Topic 3: Atomic Combinations (Molecular Structure)
Questions
MULTIPLE CHOICE QUESTIONS
1.1 A covalent bond will form between two atoms when:
A There is repulsion between the nuclei of two approaching atoms
B There is attraction between the electrons of two approaching atoms
C One atoms transfers an electron to another atom
D The potential energy of the atoms is at its lowest
(2)
1.2 In VSEPR theory, the greatest amount of repulsion between orbitals will be between:
A lone pair and lone pair
B bonding pair and lone pair
C bonding pair and bonding pair
D bonding pair and central atom
(2)
1.3 The HBr molecule forms a dipole because
i.
The Br atom is more electronegative than the H atom
ii. The H atom is more electronegative than the Br atom
iii. The HBr molecule is polar
A only (i) is true
B both (i) and (iii) are true
C both (ii) and (iii) are true
D only (ii) is true
(2)
Study the energy diagram below which represents the formation of a covalent bond
between two atoms. Use this diagram to answer questions 1.4 and 1.5
Potential energy
of the system
(kJ)
Bond length (nm)
-1,00
0,100
0,200
0,300
0,400
-2,00
-3,00
-4,00
1.4 The bond energy of the of the chemical bond is ...
A 0kJ
B 30kJ
C 330kJ
D 200kJ
24
(2)
Grade 11
PHYSICAL SCIENCES
Assessment Booklet
Topic 3: Atomic Combinations (Molecular Structure)
Questions
1.5 The bond length between the two atoms forming the chemical bond is ...
A 0,200nm
B 0,370nm
C 0,240nm
D 0,050nm
(2)
1.6 Using the table alongside, what is
Bond
Bond energy (kJ.mol-1)
C–C
348
C=C
619
C=C
835
CH4 + 2O2  CO2 + 2H2O
C-H
412
A 1891 kJ.mol-1
C=O
799
O-H
463
O=O
499
the total energy required to break the
bonds of the reactants when methane
reacts with oxygen to form carbon
dioxide and water.
B 2340 kJ.mol-1
C 1346 kJ.mol-1
(2)
D 3450 kJ.mol-1
LONG QUESTIONS
1.
2.
Draw Lewis Structures for:
(a)
a water molecule and
(2)
(b)
a carbon dioxide molecule
(2)
2.1 Define the term electronegativity.
(2)
2.2
Consider the following molecules:
HCl
F2
CH4
H2O
NH3
SF6
(a)
Assign electronegativity values to each atom.
(6)
(b)
Calculate the electronegativity difference for each molecule.
(6)
(c)
Identify which of the molecules contain polar covalent or non- polar covalent
(d)
bonds.
(6)
Identify the shapes of each molecule using VSEPR theory.
(6)
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Topic 3: Atomic Combinations (Molecular Structure)
3.
Questions
Consider the following list of molecules:
HF
BCl3
CF4
NH3
CO2
SCl6
3.1 Draw Lewis diagrams for the molecules HF and NH3 to show the bonding that
takes place.
(4)
3.2 Which of the molecules in the list are non-polar? Explain your answer.
(6)
3.3 According to VSEPR theory, what are the shapes of molecules BCl3, CF4, NH3
and SCl6?(4)
3.4 NH3 is said to have a non-ideal shape where SCl6 is said to have an ideal shape.
What is the difference between molecules these different shape classifications?
4.
(2)
For each of the following bonding pairs, say which bond is more polar. Show all calculations
and indicate the partially positive (δ+) and partially negative (δ-) poles on each bond.
5.
(a)
C – O and C – N
(3)
(b)
P – Br and P – Cl
(3)
(c)
C – O and C – S
(3)
Ammonia (NH3) is able to make a dative covalent bond with a hydrogen ion (H+) forming
the ammonium cation (NH4+).
6.
5.1 Draw the Lewis diagram for a molecule of ammonia.
(2)
5.2 Explain why the hydrogen ion can form a dative covalent bond with ammonia.
(2)
5.3 Draw a Lewis diagram of the resulting ammonium cation.
(3)
6.1 Define the term ‘bond energy’.
(2)
6.2
What is the relationship between bond energy and bond length in a stable molecule? (2)
6.3
Consider the table of bond energies and bond lengths below. Use the table to
answer the following questions:
Bond
Bond energy (kJ.mol-1)
Bond length (pm)
Cl - Cl
242
199
C-C
347
540
C=C
614
340
C=C
839
120
I-I
151
266
(a) Cl2 and I2 are both examples of diatomic halogen molecules (Group 17).
Explain why I2 has a smaller bond energy than Cl2.(2)
(b) Explain why I2 has a larger bond length compared to Cl2 .
(2)
(c) Explain why the bond energies of the different carbon to carbon bonds differ
from single to triple bonds
26
Grade 11
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(3)
Assessment Booklet
Topic 3: Atomic Combinations (Molecular Structure)
7.
Questions
Consider the reaction between oxygen and hydrogen which is used to form water
O2 + 2H2  2 H2O
Using the table of bond energies supplied, calculate:
8.
Bond
Bond energy (kJ.mol-1)
O=O
498
O-H
463
H-H
436
7.1
The amount of energy required to break the bonds in the reactants.
(3)
7.2
The amount of energy released on formation of bonds of products.
(2)
7.3
The total amount of energy released from the reaction in the formation of H2O(2)
Consider the table below. Redraw this table and complete in the space provided.
Chemical
formula of
compound
Electronegativity
difference
Non-polar
covalent, polar
covalent or
ionic bond
Shape of
molecule
Type of
molecule
(polar or
non-polar)
Cl2
CF4
CO2
HBr
NH3
H 2O
BF3
(28)
Grade 11
PHYSICAL SCIENCES
Assessment Booklet
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